I celebrate the pregnancies and births of my girlfriends babies but I don't have a sister. I lost my baby Ellen at 41 weeks. I struggled when two friends who are close got pg before I got pg again but I was still supportive. One of those friends however barely spoke to me when I was pg with Ellen because she was having trouble getting pg.
I guess there is no real answer. Everyone copes with their emotions after a loss differently and some hang on to things that are not entirely emotionally healthy. Most angel mums like your sister do sort themselves out eventually however.